The Structure of a Langmuir Monolayer of Methyl Eicosanoate as Determined by X‐Ray Diffraction and Brewster Angle Microscopy

Abstract

Relaxed Langmuir monolayers of methyl eicosanoate were studied using Brewster angle microscopy (BAM) and grazing incidence x‐ray diffraction (GIXD). The structure of the various phases in this system was determined and compared to previous isotherm and microscopy measurements. At low pressure and low temperature, a crystalline phase with tilt toward nearest neighbor, I(\(L^{\prime\prime}_{2}\)), is observed. At a temperature of \(\sim\)14\(^\circ\)C and low pressure there is a transition to a phase with tilt toward next‐nearest neighbor, F(\(L^{\prime}_{2}\)). Finally, as the temperature continues to be raised at low pressure, there is a transition to a phase where the two F(\(L^{\prime}_{2}\)) peaks have the same values of \(Q_{xy}\), here called the \(\tau\) phase, with a Rotator IV‐like structure. At high pressure and low temperature, an untilted, orthorhombic phase, \(U^{\prime}\)(CS), is observed. As the temperature is increased, a second untilted, orthorhombic phase, U(S), is observed, beginning at \(\sim\)12\(^\circ\)C. Above 21\(^\circ\)C and at high pressure, a Rotator‐II‐like phase is seen, with an undistorted, untilted structure. Finally, results from BAM measurements are used to monitor the texture of the film in the different phases.